The MRI apparatus is an apparatus which measures a nuclear magnetic resonance (hereinafter, referred to as NMR) signal generated by an object, especially, the spin of nuclei which form human tissue and images the morphology or functions of the head, abdomen, limbs, and the like in a two-dimensional manner or in a three-dimensional manner. In the imaging, different phase encoding is given to NMR signals by the gradient magnetic field and frequency encoding is also given to the NMR signals, and the NMR signals are measured as time-series data. The measured NMR signals are reconstructed as an image by a two-dimensional or three-dimensional Fourier transform.
In the above-described MRI apparatus, in order to induce a nuclear magnetic resonance phenomenon, a high-frequency magnetic field pulse (hereinafter, referred to as an “RF pulse”) which is an electromagnetic wave is emitted to an object. Therefore, as one of the clinical safety standards at the time of using an MRI apparatus, there is electromagnetic wave energy absorbed by an object. According to IEC60601-2 -33, 2nd edition, the amount of absorption of electromagnetic wave energy per unit time and unit mass is set as SAR (Specific Absorption Rate) and is defined by the following Expressions, and SAR is restricted so that the object is not irradiated with an electromagnetic wave equal to or greater than the upper limit.
                                              ⁢                              Whole            ⁢                                                  ⁢            body            ⁢                                                  ⁢            S            ⁢                                                  ⁢            A            ⁢                                                  ⁢                          R              ⁢                                                          [                              W                ⁢                                  /                                ⁢                kg                            ]                                =                                    W              ⁡                              [                W                ]                                                    M              ⁡                              [                kg                ]                                                                        (        1        )                                                          ⁢                              Body            ⁢                                                  ⁢            part            ⁢                                                  ⁢            S            ⁢                                                  ⁢            A            ⁢                                                  ⁢                          R              ⁢                                                          [                              W                ⁢                                  /                                ⁢                kg                            ]                                =                                                    W                p                            ⁡                              [                W                ]                                                                    M                p                            ⁡                              [                kg                ]                                                                        (        2        )                                          Local          ⁢                                          ⁢          S          ⁢                                          ⁢          A          ⁢                                          ⁢                      R            ⁢                                                  [                          W              ⁢                              /                            ⁢              kg                        ]                          =                  Energy          ⁢                                          ⁢          per          ⁢                                          ⁢          unit          ⁢                                          ⁢          time          ⁢                                          ⁢          absorbed          ⁢                                          ⁢          by          ⁢                                          ⁢          an          ⁢                                          ⁢          arbitrary          ⁢                                          ⁢          10          ⁢                                          ⁢          g                                    (        3        )            
Here, the whole body SAR is calculated by dividing electromagnetic wave energy absorbed by the whole body of the object by the mass of the object, the body part SAR is calculated by dividing electromagnetic wave energy absorbed by a target portion of the object by the mass of the target portion of the object, and the local SAR is electromagnetic wave energy per unit time absorbed by an arbitrary 10 g.
PTL 1 discloses a method of comparing the body part SAR calculated from the physical size information of the object with its upper limit and prompting the operator to change the imaging conditions when the body part SAR exceeds the upper limit.
In addition, PTL 2 discloses a method of calculating an optimal upper limit of the body part SAR not from an imaging part but from a body part to which an RF pulse is actually emitted.